1 files changed, 170 insertions, 0 deletions
diff --git a/gnulib-tests/test-rwlock1.c b/gnulib-tests/test-rwlock1.c
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+++ b/gnulib-tests/test-rwlock1.c
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+/* Test of glthread_rwlock_rdlock function.
+   Copyright (C) 2017-2023 Free Software Foundation, Inc.
+
+   This program is free software: you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation, either version 3 of the License, or
+   (at your option) any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program.  If not, see <https://www.gnu.org/licenses/>.  */
+
+/* Written by Bruno Haible <bruno@clisp.org>, 2005.
+   Inspired by
+   https://github.com/linux-test-project/ltp/blob/master/testcases/open_posix_testsuite/conformance/interfaces/pthread_rwlock_rdlock/2-2.c
+   by Intel Corporation.  */
+
+#include <config.h>
+
+#if USE_ISOC_THREADS || USE_POSIX_THREADS || USE_ISOC_AND_POSIX_THREADS || USE_WINDOWS_THREADS
+
+#include "glthread/lock.h"
+
+#include <errno.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <unistd.h>
+
+#include "glthread/thread.h"
+
+/* Verify that in a situation where
+     - an rwlock is taken by a reader and has a writer waiting,
+     - an additional reader requests the lock,
+     - the waiting writer and the requesting reader threads have the same
+       priority,
+   the requesting reader thread gets blocked, so that at some point the
+   waiting writer can acquire the lock.
+   Without such a guarantee, when there a N readers and each of the readers
+   spends more than 1/Nth of the time with the lock held, there is a high
+   probability that the waiting writer will not get the lock in a given finite
+   time, a phenomenon called "writer starvation".
+   Without such a guarantee, applications have a hard time avoiding writer
+   starvation.
+
+   POSIX:2008 makes this requirement only for implementations that support TPS
+   (Thread Priority Scheduling) and only for the scheduling policies SCHED_FIFO
+   and SCHED_RR, see
+   https://pubs.opengroup.org/onlinepubs/9699919799/functions/pthread_rwlock_rdlock.html
+   but test verifies the guarantee regardless of TPS and regardless of
+   scheduling policy.  */
+
+#define SUCCEED() exit (0)
+#define FAILURE() exit (1)
+#define UNEXPECTED(n) (fprintf (stderr, "Unexpected outcome %d\n", n), abort ())
+
+/* The main thread creates the waiting writer and the requesting reader threads
+   in the default way; this guarantees that they have the same priority.
+   We can reuse the main thread as first reader thread.  */
+
+static gl_rwlock_t lock;
+static gl_thread_t reader1;
+static gl_thread_t writer;
+static gl_thread_t reader2;
+static gl_thread_t timer;
+/* Used to pass control from writer to reader2 and from reader2 to timer,
+   as in a relay race.
+   Passing control from one running thread to another running thread
+   is most likely faster than to create the second thread.  */
+static gl_lock_t baton;
+
+static void *
+timer_func (void *ignored)
+{
+  /* Step 13 (can be before or after step 12):
+     The timer thread takes the baton, then waits a moment to make sure
+     it can tell whether the second reader thread is blocked at step 12.  */
+  if (glthread_lock_lock (&baton))
+    UNEXPECTED (13);
+  usleep (100000);
+  /* By the time we get here, it's clear that the second reader thread is
+     blocked at step 12.  This is the desired behaviour.  */
+  SUCCEED ();
+}
+
+static void *
+reader2_func (void *ignored)
+{
+  int err;
+
+  /* Step 8 (can be before or after step 7):
+     The second reader thread takes the baton, then waits a moment to make sure
+     the writer thread has reached step 7.  */
+  if (glthread_lock_lock (&baton))
+    UNEXPECTED (8);
+  usleep (100000);
+  /* Step 9 omitted.  */
+  /* Step 10: Launch a timer, to test whether the next call blocks.  */
+  if (glthread_create (&timer, timer_func, NULL))
+    UNEXPECTED (10);
+  /* Step 11: Release the baton.  */
+  if (glthread_lock_unlock (&baton))
+    UNEXPECTED (11);
+  /* Step 12: The second reader thread requests the lock.  */
+  err = glthread_rwlock_rdlock (&lock);
+  if (err == 0)
+    FAILURE ();
+  else
+    UNEXPECTED (12);
+}
+
+static void *
+writer_func (void *ignored)
+{
+  /* Step 4: Take the baton, so that the second reader thread does not go ahead
+     too early.  */
+  if (glthread_lock_lock (&baton))
+    UNEXPECTED (4);
+  /* Step 5: Create the second reader thread.  */
+  if (glthread_create (&reader2, reader2_func, NULL))
+    UNEXPECTED (5);
+  /* Step 6: Release the baton.  */
+  if (glthread_lock_unlock (&baton))
+    UNEXPECTED (6);
+  /* Step 7: The writer thread requests the lock.  */
+  if (glthread_rwlock_wrlock (&lock))
+    UNEXPECTED (7);
+  return NULL;
+}
+
+int
+main ()
+{
+  reader1 = gl_thread_self ();
+
+  /* Step 1: The main thread initializes the lock and the baton.  */
+  if (glthread_rwlock_init (&lock))
+    UNEXPECTED (1);
+  if (glthread_lock_init (&baton))
+    UNEXPECTED (1);
+  /* Step 2: The main thread acquires the lock as a reader.  */
+  if (glthread_rwlock_rdlock (&lock))
+    UNEXPECTED (2);
+  /* Step 3: Create the writer thread.  */
+  if (glthread_create (&writer, writer_func, NULL))
+    UNEXPECTED (3);
+  /* Job done.  Go to sleep.  */
+  for (;;)
+    {
+      sleep (1);
+    }
+}
+
+#else
+
+/* No multithreading available.  */
+
+#include <stdio.h>
+
+int
+main ()
+{
+  fputs ("Skipping test: multithreading not enabled\n", stderr);
+  return 77;
+}
+
+#endif